I pretty much use both; it depends on the situation. I don't have any hang-ups regarding the use of "I'm bipolar" because the illness is a part of me, but I will often use "I have bipolar disorder" when I'm talking with medical people who don't know me well. It's a dignity thing then---I don't wish to be treated like a "mental patient" when I'm not getting care for that reason. Weird, I know, but that's just me.